In a PVD chamber, films of material may be deposited on a substrate during processing. For example, titanium nitride (TiN) films may deposited by radio frequency (RF) plasma (13 or 40 MHz, 1-10 kW) sputtering at pressure (50-150 mT) and low sheath voltage (<50 V) at the wafer. Process parameters and operating environment is typically control to provide PVD films of good density but low stress. Controlling the sheath voltage is important to minimize the film stress. High sheath voltage over-densifies the film and creates a compressive film stress. Low sheath voltage minimizes this tendency towards over-densification.
In some PVD processing chambers, RF capacitive tuning circuits have been used to increase the substrate potential to a positive DC value. This has the by-product of also increasing the DC potential of the plasma. Since the chamber shields are at ground potential, this voltage difference increases the likelihood to sputter material from the shields and contaminate the plasma, and therefore the substrate. This problem becomes more apparent at higher frequencies such as 40 MHz and up, and when shield areas are reduced, thereby concentrating the field.
Accordingly, the inventor has provided an improved apparatus and methods for a controlling the sheath voltage by raising or lowering the overall plasma potential.